The present invention relates to an apparatus, e.g. a radiophone, to be employed in underground facilities such as mines, for providing two-way communication wherein the utilized mode of propagation of the radio waves is based, particularly with long communication distances, on the connecting of the electromagnetic waves to the metal structures, pipeworks and other conducting bodies installed in the mine galleries and shafts.
The basic problem in the underground employment of radiophones is the strong attenuation of electromagnetic waves under these conditions. This means that such radiophones that function perfectly well above ground do not function in a satisfactory way or in any way without special measures when they are used underground. Other environmental circumstances may also set additional requirements to the equipment employed.
A radiophone connection to a mine can be established in three different ways:
1. By employing the waveguide mode of propagation with frequencies of over 200 MHz; PA1 2. By employing leaky feeders with frequencies 2-200 MHz; and PA1 3. By employing propagation directly through the bedrock and based on the conducting bodies installed in the mines, in which case the frequency band 0.1-2 MHz may be possible.
The waveguide mode is a mode of propagation for electromagnetic waves, wherein the wave motion proceeds as if within a guide. The mine gallery can be considered as a kind of a waveguide. However, the attenuation of the signal is remarkable, and the obstacles--such as vehicles--located in the gallery cause additional attenuation. The method is not suited for the complex network of galleries in existing mines where the communication distances are long.
In the leaky feeder method, a long pair of cables within the same shield, or a loosely braided coaxial transmission line, is coupled to the radio transmitter. The field emitted by the transmission line can be captured by a portable receiver from a distance of a few tens of meters at the most with respect to the line. The antenna cable is installed in the mine galleries, and the use of portable equipment is possible in the vicinity of this antenna.
With a frequency below 2 MHz, such radiophone systems can be employed where the signal proceeds directly through the bedrock, or through existing conducting bodies such as power cables, pipework or hoist ropes. Direct propagation through the bedrock is dependent on the conductivity of the rock and on the employed frequency. If existing conducting bodies are utilized, the communication distances are rendered remarkably longer. The same is achieved if the base station is directly coupled to the cable which serves as the antenna.